The quality of an audio device can be determined either subjectively or objectively. Subjective tests are time consuming, expensive, and difficult to reproduce. Therefore, several methods have been developed to measure the quality of an output signal, in particular a speech signal, of an audio device in an objective way. In such methods, the speech quality of an output signal as received from a speech signal processing system is determined by comparison with a reference signal.
A current method that is widely used for this purpose is the method described in ITU-T Recommendation P.862 entitled “Perceptual evaluation of speech quality (PESQ): An objective method for end-to-end speech quality assessment of narrow-band telephone networks and speech codecs”. In ITU-T recommendation P.862, the quality of an output signal from a speech signal processing system, which signal is generally distorted, is to be determined. The output signal and a reference signal, for example the input signal of the signal processing system, are mapped on representation signals according to a psycho-physical perception model of the human auditory system. Based on these signals, a differential signal is determined which is representative of a distortion within the output signal as compared to the reference signal. The differential signal is then processed in accordance with a cognitive model, in which certain properties of human hearing perception based on testing have been modeled, to obtain a quality signal that is a measure of the quality of the auditive perception of the output signal.
Generally, objective measurement systems like PESQ interpret noise in terms of a decrease in quality. However, this interpretation is too simplistic. In present-day telecommunications systems, in particular systems using Voice-Over-IP (VOIP) and similar technologies, the impact of noise on speech quality varies in dependence of local signal level. Consequently, PESQ frequently does not provide optimum predictions of the perception of speech signals processed in such telecommunication systems, which are becoming increasingly popular.